Sinusoidal Intensity Modulation Research Articles

Nonlinear dynamics of relative intensity noise transfer at fiber amplification and supercontinuum generation

Supercontinuum sources via femtosecond pumping in highly nonlinear photonic crystal fibers considerably impact scientific research and industrial applications. Herein, we investigate the complex transfer function of relative intensity noise in a broadband fiber supercontinuum source generated using an ytterbium-doped mode-locked laser. A designed intensity noise was artificially added to the system through the introduction of weak sinusoidal intensity modulation to the seed pulses. Owing to the influence of nonlinear gain dynamics and soliton dynamics during power amplification and spectral broadening, the transfer functions exhibit unique frequency and wavelength dependencies. Systematic measurements indicate that the seed transfer functions exhibit the characteristics of damped high-pass filters and saturate at low Fourier frequencies within the fiber amplifier. Moreover, supercontinuum broadening induces a nonlinear transformation of fluctuations, resulting in considerable amplification and negative correlation within the dispersion wave and soliton self-frequency shift regions. Clarifying the mechanism and bandwidth of this system is imperative to achieving low-noise supercontinuum generation because they provide insights into the nonlinear laser dynamics associated with broadband spectral broadening and noise transformation.

Experiments on an AM Mode-Locked Laser as an Arbitrary Optical Function Generator

We show experimentally that various optical pulses can be generated from an AM mode-locked laser by employing a 10 GHz harmonically and regeneratively mode-locked erbium fiber laser. A software-controlled liquid crystal on silicon (LCoS) optical filter device, which can control the amplitude and phase of the input signal as a function of optical frequency, was employed to implement a specific filter function. The filter was characterized by the Fourier transformed spectral profile <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$A(\omega )$ </tex-math></inline-formula> of the output pulse <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$a(t)$ </tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$A(\omega +\Omega _{m}), $ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$A(\omega -\Omega _{m})$ </tex-math></inline-formula> . Here, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Omega _{m}$ </tex-math></inline-formula> is the AM modulation angular frequency. The filter resolution was set at 10 GHz. By installing a specific filter in the laser cavity, we succeeded in generating various pulses such as Gaussian, secant hyperbolic (sech), double-sided and single-sided exponential, parabolic, triangular, and Nyquist pulses with an arbitrary roll-off factor as with an optical function generator. It is interesting to note that we could experimentally generate very precise transform-limited (TL) optical pulses, which were originally designed by calculating the filter function. The generated spectral profile agreed well with the numerical results even below −40 dB, which also agreed with the theory. As we have shown theoretically, we could generate sech and parabolic pulses without fiber nonlinearity. An asymmetric single-sided pulse can also be generated by incorporating a complex filter function with phase information, where a small amount of phase rotation of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim \pi $ </tex-math></inline-formula> /60 in each round trip played an important role in generating the asymmetric pulse. Without the phase change, the pulse became symmetric and was no longer the designed pulse. A clean triangular pulse was difficult to generate since parabolic pulse shaping on the top of a sinusoidal intensity modulation is ineffective in creating a sharp peak.

All-optical enhancement of minimum detectable perturbation in intensity-based fiber sensors.

We present a novel optical signal processing scheme for enhancing the minimum detectable environmental perturbation of intensity-based fiber sensors. The light intensity is first stabilized by inducing a sinusoidal intensity modulation and extracting the first-order sideband generated by self-phase modulation (SPM) in a nonlinear medium. The light with stabilized intensity is then sent through a sensor and the sensor induced power variation is magnified by first inducing a sinusoidal intensity modulation, then undergoing SPM, and finally extracting a higher-order sideband. The advantage of the proposed stabilization-magnification (SM) sensing scheme is experimentally demonstrated by applying a damped vibration on an intensity-based fiber sensor and comparing the minimum detectable strain value of the proposed scheme with that of a conventional sensing scheme. Experimental results demonstrate minimum detectable strain improvement by a factor of 3.93. This new SM sensing scheme allows for the detection of perturbations originally too weak to be detected by a given intensity-based fiber sensor, which will be beneficial for a variety of applications such as high frequency ultra-sound detection.

Geometric scattering removal in cross-correlation Doppler global velocimetry by structured illumination

The biases introduced by background light contributions in planar, Doppler-based velocimetry are significantly reduced by the application of structured illumination. The method works by measuring and separating desired flow-scattered signals from unwanted reflections and secondary scattering contributions via a sinusoidal intensity modulation across the laser sheet used to interrogate the flow. In the current work we consider the influence of background light on cross-correlation Doppler global velocimetry (CC-DGV) and present the results for measurements in a turbulent free jet with an exit Mach number of 0.5 in a measurement arrangement with high intensity background contributions. It is shown that if the ratio of the particle-scattering (i.e. Mie scattering) intensity to the background solid surface scattering (i.e. geometric scattering) intensity is greater than 10, then the measured signal displays a significant bias away from the true velocity. With structured illumination, the measured intensity corresponding to geometric scattering is reduced to negligible levels, reminiscent of a measurement arrangement with little or no background light influence. Centreline velocity measurements with this background subtraction method are compared with Kiel-type pressure probe measurements, yielding a root-mean-square (RMS) difference between the two of 1.2 m s-1 (less than 1% of the full scale velocity), well within the published uncertainty bound of the CC-DGV technique, underscoring the robustness of this approach. In addition to the centreline validation, the streamwise vorticity and total pressure are also analysed, showing trends indicative of the S-type shape of the duct and aggressive flow entrainment experienced in this environment.

Neutron MIEZE spectroscopy with focal length tuning

We report on a method to tune the focal length of high resolution neutron MIEZE spectrometers. The MIEZE technique relies on a fast sinusoidal neutron intensity modulation up to the MHz range, generated by the rotation of the neutron spin in radio-frequency spin flippers, and subsequent conversion to an intensity modulation by a spin analyzer. This intensity modulation is washed out due to the neutron velocity dispersion, but by proper choice of the spin rotation frequencies as well as the distances between sample, detector and spin flippers, a focal point in space appears (echo point), where the neutron detector is placed. In this work, we describe how to extend the dynamic range of the MIEZE technique by several orders of magnitude by introducing a field subtraction coil (NSE coil), such that at low energy resolution a good overlap with conventional spectroscopy techniques is achieved. All formulas for calculating the tuning parameters and an experimental example from the RESEDA spectrometer at the Heinz Maier–Leibnitz Zentrum (MLZ) are discussed.

Optical vortex with a small core and Gaussian intensity envelope for light-matter interaction

Optical vortices with a vortex core size that is at least two orders of magnitude smaller than the laser beam waist is presented. The optical vortex is generated by a spiral phase plate (SPP) and counterrotating optical vortex pairs are created in a modified Mach–Zehnder interferometer surrounded by a 4f lens arrangement. The azimuthal variation of the counterrotating optical vortex forms a sinusoidal intensity modulation for which the winding number of the optical vortex is deduced accurately and precisely by fitting it to a sinusoidal function. These results are of interest in designing novel optical vortex gratings for manipulating matter waves (e.g., in Kapitza–Dirac scattering). A theory of atomic angular Kapitza–Dirac scattering with optical vortices is presented. The large beam waist combined with a small optical vortex core size would also be of interest when using an optical vortex to perform spectroscopy in a wide range of matter systems including solid, liquid, atomic, and molecular systems, as well as in short-range optical communication.

Line scanning time-of-flight laser sensor for intelligent transport systems, combining wide field-of-view optics of 30 deg, high scanning speed of 0.9 ms / line , and simple sensor configuration

We developed a line scanning time-of-flight (TOF) laser sensor for an intelligent transport system (ITS), which combines wide field-of-view (FOV) receiving optics of 30 deg and a high-speed microelectro mechanical system scanner of 0.9 ms/line with a simple sensor configuration. The newly developed high-aspect ratio photodiode realizes the scanless and wide FOV receiver. The sinusoidal wave intensity modulation method is used for the TOF measurement. This enables the noise reduction of the trans-impedance amplifier by applying the LC-resonant method. The vehicle detection and axle counting, which are the important functions in ITS, are also demonstrated.

A Swift view on IGR J19149+1036

IGR J19149+1036 is a high-mass X-ray binary detected by INTEGRAL in 2011 in the hard X-ray domain. We have analysed the Burst Alert Telescope (BAT) survey data of the first 103 months of the Swift mission detecting this source at a significance level of ∼30 standard deviations. The timing analysis on the long-term BAT light curve reveals the presence of a strong sinusoidal intensity modulation of 22.25 ± 0.05 d, that we interpret as the orbital period of this binary system. A broad-band (0.3–150 keV) spectral analysis was performed combining the BAT spectrum and the X-Ray Telescope (XRT) spectra from the pointed follow-up observations. The spectrum is adequately modelled with an absorbed power law with a high-energy cutoff at ∼24 keV and an absorption cyclotron feature at ∼31 keV. Correcting for the gravitational redshift, the inferred magnetic field at the neutron star surface is Bsurf ∼ 3.6 × 1012 G.

Rocking motion of an optical wing: theory.

The intensity-dependent rocking frequency of an illuminated semicylindrical refractive rod (or "optical wing") on a flat, nonslip surface is investigated. Both longitudinal and transverse radiation pressure forces (scatter and lift forces), as well as radiation pressure torque, transform the mechanical system into one having a bistable potential energy above a critical intensity. The equation of motion may be written as a parametrically driven nonlinear bistable harmonic oscillator, resulting in complex rocking dynamics. The effects of linear and sinusoidal intensity modulation schemes are explored, and experimental conditions to verify these results are discussed.

Pulse dynamics in a mode-locked fiber laser and its quantum limited comb frequency uncertainty.

We present an experimental study of pulse dynamics in a mode-locked Er:fiber laser. By injecting a continuous wave laser with sinusoidal intensity modulation into the fiber laser, we are able to modulate the gain. Measuring the response of the pulse energy, central frequency, central pulse time, and phase to the gain modulation allows determination of the parameters that describe their coupling. Based on the experimentally derived parameters, we evaluate the free running comb linewidth and frequency uncertainty with feedback included, assuming quantum noise is the limiting factor. Optimization of fiber lasers is also discussed.

Optical microfiber phase modulator directly driven with low-power light

An optical microfiber phase modulator (OMPM) directly driven with low-power light is presented. Phase modulation response of OMPM is theoretically analyzed. A 10-mm optical microfiber (OM), tapered from conventional single-mode fiber, is inserted in one arm of a Michelson fiber interferometer. To drive the OMPM, 980-nm wavelength light with sinusoidal intensity modulation is injected into the interferometer. The OMPM response properties are measured and p-phase modulation amplitude can be obtained with only 7.5-mW average power light at 1-kHz modulation frequency. The OMPMs shown in this study have advantages of simple structure, potential compact size, and low-power-driven light.

Highly Accurate Compensation Technique for 10-GHz Pulse Intensity Fluctuation Using SPM-Based All-Optical Intensity Limiter

We report a trial of 10-GHz and highly accurate pulse-by-pulse intensity limiting with a single erbium-doped fiber amplifier for low-power-consumption driving optical limiting. A self-phase modulation-based optical limiter with an appropriate pre-chirping made it possible, which provides high limiting accuracy of less than 0.1 dB at a 10-GHz bit rate. The limiting function was tested in a simulation and an experiment for 10-GHz bit sequence with intensity fluctuation. When fed a 1.6-dB sinusoidal intensity modulation at 2 GHz, the experimental setup successfully achieved optical limiting with 0.1-dB accuracy.

Modeling and Analysis of Microring Resonator Modulators With Feedback Waveguide Coupling

The model and dynamic numerical solution of microring resonator modulators with feedback waveguide coupling are proposed and used to analyze the modulation performance. The sinusoidal intensity modulation output and bandwidth are presented. The feedback arm modulation manner is potential to ensure stable output with frequency increasing and wide bandwidth. Phase characteristics studies show that the resonator is suitable for phase shift keying. Furthermore, the Gaussian pulse modulation is studied. Results show that linear and undistorted output can be achieved for narrow and high speed pulse modulation while the resonator is biased at critical coupling position.

ϕ^2FLIM: a technique for alias-free frequency domain fluorescence lifetime imaging

A new approach to alias-free wide-field fluorescence lifetime imaging in the frequency domain is demonstrated using a supercontinuum source for fluorescence excitation and a phase-modulated image intensifier for detection. This technique is referred to as phi-squared fluorescence lifetime imaging (phi(2)FLIM). The phase modulation and square-wave gating of the image intensifier eliminate aliasing by the effective suppression of higher harmonics. The ability to use picosecond excitation pulses without aliasing expands the range of excitation sources available for frequency-domain fluorescence lifetime imaging (fd-FLIM) and improves the modulation depth of conventional homodyne fd-FLIM measurements, which use sinusoidal intensity modulation of the excitation source. The phi(2)FLIM results are analyzed using AB-plots, which facilitate the identification of mono-exponential and multi-exponential fluorescence decays and provide measurements of the fluorophore fractions in two component mixtures. The rapid acquisition speed of the technique enables lifetime measurements in dynamic systems, such as temporally evolving samples and samples that are sensitive to photo-bleaching. Rapid phi(2)FLIM measurements are demonstrated by imaging the dynamic mixing of two different dye solutions at 5.5 Hz. The tunability of supercontinuum radiation enables excitation wavelength resolved FLIM measurements, which facilitates analysis of samples containing multiple fluorophores with different absorption spectra.

A binaural advantage in the subjective modulation transfer function with simple impulse responses

The speech transmission index (STI) has been a popular method for predicting speech intelligibility in rooms. It is based on the magnitude of the modulation transfer function, which can be derived from the impulse response of the room and the background noise levels. However, it does not take into account that humans listen with two ears. There can be large interaural phase differences in the modulation transfer functions, which can create detectable interaural level difference fluctuations. Measurements were made to determine whether these interaural modulation phase differences can be used to enhance the detectability of sinusoidal intensity modulations imposed on a broadband noise carrier and then convolved with simple, dichotic impulse responses. The results show that there can be a significant advantage to listening with two ears over listening with just one. Some further investigations were made to determine whether this advantage in signal detection can also be used as an advantage for speech intelligibility.

Automatic Full-Field 3-D Profilometry Using White Light Confocal Microscopy with DMD-Based Fringe Projection

A new full-field 3-D micro surface profilometer using digital micromirror device (DMD)-based fringe projection strategy and confocal principle is presented in the article. In viewing the fact that conventional laser confocal measurement method not only easily encounters undesired irregular scattering problems, but also lack scanning efficiency due to its single-point type measurement, the newly developed automatic surface profilometer deploys a DMD chip to project spatially encoded digital fringe patterns with dynamic light intensity, onto the object to obtain excellent measurement performance. A novel digital fringe pattern design with adaptive sinusoidal intensity modulation was developed for active fringe projection, to obtain optimized depth resolution with a micrometer lateral resolution in confocal measurement. Some of semiconductor components have been measured to attest the feasibility of the developed approach. The depth measurement resolution can reach better than 0.1μm and the maximal measured error was verified to be less than less than 0.5 % of the measured step size.

Temporal resolution in the eyes of marine decapods from coastal and deep-sea habitats

Temporal responses of eyes from four decapod species taken from sublittoral (Pandalus montagui Leach), coastal [Nephrops norvegicus (L.)] and deep-sea [Paromola cuvieri (Risso) and Chaceon (=Geryon) affinis A. Milne Edwards and Bouvier] habitats were examined. The electroretinogram responses to a range of sinusoidal intensity modulations between 0.5 and 40 Hz were recorded. Recordings were made from individuals adapted successively to two background light intensities. The sublittoral and coastal species showed faster responses when adapted to the higher light level and they also responded best to intermediate frequencies. When adapted to the lower light level, all species responded most strongly to low-frequency stimuli. Physiological and ecological reasons for the differences in responses are suggested.

Observation of Rydberg wave packet dynamics in a Coulombic and magnetic field.

Rydberg electron wave packets are investigated, using phase-modulated detection, In the absence of any external field we observe pure radial motion of a bound electron wave packet showing both the first integer revival and all fractional revivals up to the seventh order; in the presence of a magnetic field (0.81 T) both paramagnetic and diamagnetic effects are observed. Paramagnetism is visible in the time-correlation function of the wave packet as a sinusoidal intensity modulation, with a period of 43 ps, superimposed on the radial motion; the diamagnetism is observed as a decrease in the radial recurrence times (by up to 35%).

Filament formation in a tapered GaAlAs optical amplifier

Filament formation in a tapered GaAlAs amplifier with an output width of 320 μm is characterized by injecting an input beam with a superimposed sinusoidal intensity modulation (30 μm period, 30% peak-to-peak modulation). Strong seeded filamentation of the output near field, and large far field side lobes are observed for powers above approximately 1 W. Time dependent decay of the main far field lobe, characterized by a time constant of several microseconds, is used to separate the effects of localized carrier density variations from the thermal contribution to intensity and phase distortion of the output beam.

Light adaptation and frequency transfer properties of cat horizontal cells

The frequency transfer properties of horizontal cells in the cat retina were studied as a function of the mean light intensity level and stimulus contrast. To this end, horizontal cell responses to sinusoidally modulated light stimuli were recorded intracellularly in the optically intact, in vivo eye. The light stimulus consisted of a 3.9 deg dia. spot superimposed on a steady background (8.8 deg dia.). A discrete Fourier analysts was performed in order to describe the amplitude and phase characteristics of the linear response component and in order to specify the nonlinear distortion of the response. The amplitude of the fundamental Fourier component was found to increase linearly with the contrast of the sinusoidal light intensity modulation. Increasing the mean light level while keeping the contrast constant caused a frequency dependent increase in response amplitude. The increase was most pronounced at high temporal frequencies and resulted in a conspicuous increase of the flicker fusion frequency. Steady background illumination caused a reduction of the response amplitudes at the lower temporal frequencies. Responses in the high frequency range, however, were not affected. The phase shifts of the fundamental Fourier components were found to diminish at increasing mean illumination levels. The harmonic distortion of horizontal cell responses to sinewave flicker was studied as a function of stimulus frequency and stimulus contrast. By comparing the data obtained using sinusoidal light intensity modulation with the intensity profiles described in a preceding paper it was investigated to what extent the harmonic distortion can be explained by the nonlinearity expressed in the response vs intensity profiles.